The actin-based stereocilary bundle (or hair bundle) on the apex of auditory hair cells serves the crucial function of converting sound energy to electric signals. Its staircase structure renders the bundle directionally sensitive to mechanical stimuli. As such, auditory hair bundles must be uniformly oriented for correct sound transduction. Abnormalities in hair bundle structure or orientation cause deafness and hearing impairment. The uniform orientation of auditory hair bundles is a prominent example of epithelial planer cell polarity (PCP), or polarity within the plane of the epithelial sheet. It is controlled by an evolutionarily conserved Wnt/PCP signaling pathway. Disruption of the PCP pathway results in severe developmental defects including neural tube closure defect and hair bundle misorientation, underscoring its critical function in regulating tissue morphogenesis. Despite its importance, the molecular and cellular mechanisms by which the PCP pathway regulates the intricate morphogenetic processes during hair bundle development are still poorly understood. The long-term goal of this research is to understand how PCP signaling directs coordinated remodeling of the hair cell cytoskeleton to form uniformly oriented hair bundles. The primary focus of this project is to dissect the function of protein tyrosine kinase 7 (PTK7), a novel component of the mammalian PCP pathway previously identified in a mouse gene-trap screen. PTK7 encodes a catalytically inactive receptor tyrosine kinase. Mouse models, cochlear organ cultures, and a combination of genetic, biochemical and cell biological approaches will be used to test the central hypothesis that PTK7 deploys both conserved and novel mechanisms to regulate hair bundle orientation. Aim 1 will determine where PTK7 feeds into the conserved PCP pathway. The hypothesis to be tested is that PTK7, via its cytoplasmic kinase domain, interacts with the PCP signaling molecule Dishevelled to activate the small GTPase Rac, leading to cytoskeletal remodeling. Aim 2 will determine the cell-autonomy of PTK7 in the organ of Corti and the role of proteolysis in regulating intra- and inter-cellular signaling activity of PTK7. In Aim 3, novel signaling mechanisms of PTK7 will be explored, first by testing the kinase function of PTK7, followed by isolating novel PTK7 interacting proteins. Understanding PCP signaling mechanisms that control cytoskeletal remodeling during hair bundle development will aid in devising therapies to promote hair cell repair and meaningful functional recovery following repair. Moreover, because the PCP pathway regulates morphogenesis of diverse tissues, including the neural tube, studies of hair cell morphogenesis will likely provide novel insights into the general principles of PCP signaling during tissue morphogenesis, which will facilitate the diagnosis and treatment of many devastating human birth defects.